Spread spectrum techniques have proven useful in a variety of communications applications, including cellular telephones, wireless local area networks, and military communications. One advantage provided by spread spectrum techniques is the ability to build a transmitter which is difficult for an unauthorized user to detect.
Wireless spread spectrum systems operate by using a relatively large amount of spectrum bandwidth to communicate their signals. The large bandwidth is consumed by spread spectrum encoding the message data using a pseudonoise code. The two most common types of spread spectrum transmission are frequency hopping, where the pseudonoise code is used to pseudo randomly change the transmission frequency on a periodic basis, and direct sequence, where the pseudonoise code is used to modulate the transmit signal at a high rate relative to the underlying message data.
In order to detect a spread spectrum transmission, it is generally necessary to know the pseudonoise code beforehand. Furthermore, to extract the message data, it is generally necessary to know the timing of the pseudonoise code. For example, in a direct sequence system, this can be accomplished by knowing the code frequency (rate at which the pseudonoise code advances through its sequence) and the starting time of the pseudonoise code (sometimes referred to as the phase of the code). A signal for which the spread spectrum receiver knows the pseudonoise code, pseudonoise code phase, and pseudonoise code frequency can be referred to as a synchronized signal.
One interesting property of spread spectrum systems is that unsynchronized signals appear as noise to a spread spectrum decoder, and are suppressed by the decoder. This property is sometimes used to provide a so-called spread spectrum multiple access system (also known as code division multiple access). For example, different users can be assigned different pseudonoise codes, in which case a receiver will reject signals from users other than the specific user to whose code the receiver is synchronized. As another example, all users can be assigned a common pseudonoise code, but each user transmits with a different pseudonoise code start time. This results in each user having a different pseudonoise code phase. A receiver tuned to the common pseudonoise code at a particular timing (phase) will reject other user with different code timing (phase). This latter example is sometimes referred to as spread-ALOHA.
Achieving synchronization with a spread spectrum signal can be difficult, in part due to high pseudonoise code rate (frequency). For example, a relatively low message data rate of 1,000 bits per second might be spread spectrum encoded with a relatively high pseudonoise code rate of 10,000,000 chips per second, where a bit of the pseudonoise code is referred to as a chip. In this example, the ratio of 10,000,000/1,000=10,000 is the processing gain. A spread spectrum receiver for this signal will need to synchronize to the high pseudonoise code rate being used by the transmitter, and hence the spread spectrum receiver requires a factor of 10,000 higher synchronization accuracy than a non spread spectrum system. The difficulty of achieving this synchronization increases as the processing gain increases.
In order to limit the difficulty of synchronizing spread spectrum systems, various techniques have been used. These techniques include the use of very stable oscillators to generate the carrier frequency on which the transmission is centered, the use of very stable clocks to generate the pseudonoise code, and the transmission of special pilot signals or long preambles of known data to aid receiver in synchronization.
Another property of spread spectrum systems is a generally low probability of detection by a user lacking knowledge of the pseudonoise code. This is because the transmitter power of the spread spectrum signal is spread out over a relatively large portion of radio spectrum. By using a high processing gain, it is possible to sufficiently spread the transmitter power out so that the resulting transmission spectral power density is below the noise level within the environment. In general, it is more difficult to detect a spread spectrum signal without knowledge of the pseudonoise code as the processing gain is increased, making the use of high processing gain desirable. Unfortunately, higher processing gains also make acquisition of the spread spectrum signal more difficult for authorized receivers that know the pseudonoise code.
A particular challenge exists in a spread spectrum system which has a large number of transmitters, each of which operates at a relatively low data rate, yet requires a low probability of detection (and hence high processing gain and high pseudonoise code rate). For example, transmitters may be configured to periodically transmit short bursts of message data at a relatively low rate, each transmitter using a common pseudonoise code, yet starting the transmission at a unique starting time. Furthermore, very short preambles may be used, for example to limit power consumption and enhance the low probability of detection. A receiver is faced with a considerable challenge in detecting these short message transmissions which have an unknown start time. Since the message transmissions are short, there is a limited amount of time to detect the message. Traditional approaches which sequentially search a plurality of hypothesized start times can thus prove ineffective at detecting these short transmissions, since the transmission may occur while the searching is being done using a different hypothesized start time than that of the transmission.
The problem just described is further aggravated when the transmitters are designed to achieve very low cost. Hence, the oscillators used may provide relatively low accuracy and stability, resulting in carrier frequency offsets and code frequency offsets. Furthermore, the code frequency offset may be unrelated to the carrier frequency offset due to a combination of different oscillators and Doppler effects. Accordingly, a receiver is faced with a challenging problem of detecting the transmissions.